1. Field of the Invention
The present invention relates to the assembly and subassembly of an automobile. More specifically, it relates to a method for assembly of a body-in-white (BIW) of an automobile.
2. Background Art
A major goal of the automobile manufacturer is to employ an assembly process that minimizes the total dimensional variation of the finished automobile body. The total dimensional variation represents the intrinsic variation in parts, subassemblies, and materials, as well as the variation induced by the sequences and methods used during assembly. Fit and finish represents one method of characterizing the total dimensional variation of an automobile. It is a subjective measure of the quality of the assembled automobile in terms of the sizes and gaps between adjacent body surfaces and the flushness of different surfaces. One example is the spatial relationship of the hood to fender or the hood to the leaf screen. Other examples might include the leaf screen to fender, hood to grille, or fender to door relationships.
Body-in-white (BIW) is a term used in automobile assembly to describe a structurally rigid frame of a partially completed vehicle body before the powertrain, exterior trim, and interior components are installed. The BIW typically comprises the underbody, side frames, front or rear headers, roof, and the back panel. The doors, hood, deck-lid panels, windshield, and backlight (i.e., closure surfaces) are installed into the openings of the assembled BIW. Many of the assembly and securing-together steps involved in producing the BIW are automated operations. While a few bodies are still manually assembled and welded, the recent years have generated numerous automated and semi-automated framing systems. Therefore, if the dimensional variation of the BIW is improved, fit and finish of the closure panels would improve also. The automobile industry has developed standard procedures for measuring the total dimensional variation of each BIW as it is assembled.
Dimensional variability, even in the thousandths of an inch represents a continuous challenge for automobile assembly operations. Conventional manufacturers often assemble vehicles by employing a strategy of attaching one incremental part at a time. Individual components of the vehicle BIW, for instance a dash panel, might undergo preliminary sub-assembly operations as it moves between various assembly stations. The individual BIW moves in a specific sequence between individual assembly stations designed to further integrate the partially completed BIW carcass with additional vehicle components by affixing additional parts to the assembly using by welds, glue, bolts, etc. Moreover, individual BIW subassembly components may be affixed together at a sub-assembly station in the assembly process to form a rigid portion of the partially completed BIW carcass. As additional components are rigidly added to the BIW carcass, the spatial relationships, as well as the relative position between one component and another is established. By loading each of these additional components into each station's framing jig or fixture, and rigidly attaching it thereto, the carcass moves between multiple stations and fixtures and experiences a series of load-weld-load sequences. Unfortunately, each framing and fixture operation contributes to the increased dimensional variation by establishing the spatial and geometric relationship between that particular new component(s) and the rest of the carcass therefore further contributing to positional variability. Furthermore, as initial welds are covered up by subsequent sub-assembly components, the initial welds become closed off or “closed out”, making them inaccessible and unavailable for realignment.
Conventional assembly operations have employed one of three primary methods for managing the total dimensional variability of BIWs. First, they can spend more time manufacturing parts to exacting tolerances. For example, complex assembly elements can be designed and manufactured at significantly higher costs.
Secondly, assembly operations can reduce the speed of the assembly line. By spending more time or adding more labor during the assembly, a slight improvement could be predicted to ensure fit and finish quality. However, this slows down overall vehicle production and adds. significant cost.
A third alternative is to live with the assembly problems in the short term and instead wait to establish or correct the relationships of the BIW after the closure surfaces are integrated. Conventional processes currently use relationship mechanisms such as a fender setting machine in order to bend or twist the combined BIW and closure surfaces and establish relationships. Although this type of rework can make the closure surfaces cosmetically acceptable, it leaves open the opportunity for functional problems, such as squeaks and rattles, fit and finish variation such as wind noise, water leaks, and customer dissatisfaction.
A number of prior art inventions have described different methods for reducing the total dimensional variation of the BIW. Some prior art inventions have disclosed the limited use of hydroformed tubular members in front end assemblies to combine functions into a single part. For example, Gerricke et al., U.S. Pat. No. 6,416,119, describes a vehicle front end constructed using hydroformed tubes. However, these methods have yet to be integrated to reduce the number of “load-weld” sequences and ignore the total dimensional variability. Rather, this manufacturing strategy is still based on continuing the conventional load-weld-load processes.
In contrast, U.S. Pat. No. 6,360,421 to Oatridge et al., describes a method for reducing dimensional variation during the manufacturing of an automobile BIW from a plurality of components. The method comprises forming a substantially rigid structure from some of the plurality of components. For each of the remaining components, the further steps of referencing from said rigid sub-assembly a desired position from said each remaining component on said initial structure, and thereafter, affixing said each remaining component to said rigid sub-assembly at said desired position whereby the tolerance of said manufactured assembly is reduced. However, similar to conventional assembly operations, these methods have yet to be integrated to reduce the number of “load-weld-load” sequences.
What is needed is an improved method for reducing the total dimensional variation of BIWs during assembly and shifts the paradigm by reducing the number of “load-weld” sequences.